For example, the following methods have been reported for preparing fluorine-containing olefins substituted with organic groups.
Non-patent Literature 2 discloses a method for first converting a carbon-halogen (C—X) bond of CF2═CFX (X: a halogen atom other than a fluorine atom) into a carbon-lithium (C—Li) bond by butyllithium, and then performing a C—C bond forming reaction. Non-patent Literature 3, 4 and 5 disclose a method for further converting the Li of a C—Li bond formed as described above into a metal, such as Sn, Si, or the like, and then performing a C—C bond forming reaction.
However, these methods are disadvantageous in that CF2═CFX used as a raw material is relatively difficult to obtain or relatively expensive. Further, because the fluorine-containing lithium reagent having the C—Li bond formed at the first stage is very unstable, it is necessary to conduct the reaction under a low temperature of about −100° C. Therefore, these methods are not practical.
Non-patent Literature 6 to 8 disclose methods of reacting TFE with an organic lithium reagent or an organic magnesium reagent, thereby selectively substituting one fluorine atom. In the formula shown below, Ph represents an optionally substituted phenyl.PhLi+CF2═CF2→PhCF═CF2  (Non-patent Literature 6)PhMgBr+CF2═CF2→PhCF═CF2  (Non-patent Literature 7 and 8)
These methods are disadvantageous in that, in order to obtain the desired product with high selectivity, it is necessary to perform the reaction at a low temperature using a large excess of raw material TFE. When the reaction temperature increases, the reaction progress goes out of control, thereby producing, in addition to the desired product, 1,2-adducts, products with a larger number of substituents, etc. Consequently, the yield of the desired product greatly decreases. When a low nucleophilic organic lanthanide reagent is used, the yield of the desired product does not improve too (Non-patent Literature 9).
Non-patent Literature 10 discloses a method of reacting HFC134a (CF3CFH2) with alkyl lithium and generating a fluorine-containing vinyl lithium by an elimination reaction. It further discloses a coupling reaction of the fluorine-containing vinyl group via a vinyl zinc reagent generated by performing a metal exchange reaction with zinc.
However, this method is disadvantageous in that it requires an excessive amount of expensive alkyl lithium, and it also poses a difficulty in precisely controlling the reaction temperature due to the instability of the fluorine-containing vinyl lithium.
If it were possible to substitute a fluorine atom (F) on an sp2 hybridized carbon atom in the molecule with an organic group using TFE, hexafluoropropene (HFP), etc., which are readily obtainable industrially, in the presence of a transition metal catalyst, the method would be more useful for synthesizing substituted fluorine-containing olefins than the known methods described above.
Generally, there are many reports regarding the methods for introducing a substituent into a nonfluorinated olefin using a transition metal as a catalyst, but there are extremely few reports regarding the methods for conducting a reaction that activates a C—F bond in a fluorine-containing olefin and then generates a C—C bond. This is presumably because the binding energy of the C—F bond in the fluorine-containing olefin is much higher than that of the C—Y bond (Y represents Cl (chlorine), Br (bromine), I (iodine) or the like) of other halogen-containing olefins, and also because the fluorine atom, which is small and hard, makes it difficult to cleave the C—F bond or to perform an oxidative addition reaction of a metal with the C—F bond.
Recently, a method has been reported (Patent Literature 1 and Non-patent Literature 11) wherein the carbon-fluorine bond of tetrafluoroethylene (TFE) is activated using a transition metal catalyst to substitute fluorine with an organic group or groups using an organozinc reagent.
The advantage of this procedure is that the reaction conditions are mild compared to those in the methods described above, and the product selectivity is high. However, this method, is problematic in the handling of the organozinc reagent itself. More specifically, because organozinc reagents exhibit low stability with regard to temperature and humidity, the reaction needs to be conducted under an inert atmosphere. Furthermore, because it is difficult to store reagents for a long period of time, it is often necessary to prepare them at the time of use.
Organic boron reagents are often used in reactions where carbon-carbon bonding proceeds under the presence of a transition metal catalyst. These organic boron reagents exhibit low toxicity compared to other organic metal reagents, and the reagents per se are stable. Among them, boronate derivatives have remarkably advantageous characteristics, such as usability also in water. Due to such characteristics, boronate derivatives allow carbon-carbon bonding to be selectively formed at desired positions even in the presence of a hydroxy group, etc., which cannot coexist with other reagents having a high nucleophilicity, including the organozinc reagents mentioned above.
Organic boron reagents have various advantages as described above. However, the only reaction ever reported is a substitution reaction of a chlorine atom on an sp2 hybridized carbon atom in chlorotrifluoroethylene (Non-patent Literature 12). There are no reports of the use of an organic boron reagent for a substitution reaction of a fluorine atom on an sp2 hybridized carbon atom in a fluorine-containing olefin.